Material Science Test 2

Strength of metals is increased by....

1.) making dislocation motion difficult 2.) decreasing grain size 3.) solid solution strengthening 4.) cold working

Hardness

A measure of a material's resistance to localized plastic deformation (a small dent or scratch). A property that directly relates to both the materials resistance to wear and its overall tensile strength. Rockwell and Brinell are most common hardness testing techniques. Rockwell: Several scales are used; based on the difference in indentation depth from the imposition of minor and major loads. Brinell: One scale is used; determined from indentation size.

Ductility

A measure of the degree a material will plastically deform by the time fracture occurs. Ductility is measured in terms of percent elongation(a measure of the plastic strain at fracture) and percent reduction in area. With increasing temperature, ductility increases.

Case Hardening

A technique by which both surface hardness and fatigue life are enhanced for steel alloys. This is accomplished by a carburizing or nitriding process.

Ductile Fracture

Accompanied by significant plastic deformation. It gives warning before actual fracture occurs. 2 tensile fracture profiles are possible: 1.) Necking down to a point fracture when ductility is high 2.) Only moderate necking with a cup-and-cone fracture profile when the material is less ductile.

self-diffusion

Atomic migration in pure metals. For host atoms, self-diffusion is used.

interdiffusion

Diffusion of atoms of one metal into another metal. Also called impurity diffusion. (i.e. nickel and copper diffusing into one another)

Recovery

During Recovery: There is some relief of internal strain energy by dislocation motion as a result of atomic diffusion at elevated temperatures. Dislocation density decreases and energy levels are low.

Recrystallization

During recrystallization: a new set of strain-free and equally sized grains form that have relatively low dislocation densities; therefore the metal become softer, weaker, and more ductile. The driving force for recrystallization is the difference in internal energy between strained and recrystallized material. For a cold-worked metal that experiences recrystallization, as temperature increases (at constant heat-treating time), tensile strength decreases and ductility increases.

Activation Energy

Energy required to produce the diffusive motion on one mole of atoms. A large activation energy results in relatively small diffusion coefficient.

Fatigue

Failure under applied cyclic stresses. 90% of all mechanical failures are caused by fatigue. The process occurs by the initiation and propogation of cracks, and ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress. Crack Initiation: a small crack forms at some point of high stress concentration Crack Propogation: crack advances incrementally with each stress cycle

Grain Growth

Grain growth is the increase in average grain size of polycrystalline materials, which proceeds by grain boundary motion.

Brittle Fracture

Little to no plastic deformation, gives no warning before fracture which causes catastrophic events. 2 types 1.) Intergranular: cracks follow grain boundaries 2.) Transgranular: Cracks go through grain boundaries

diffusion

Mass transport within solid materials by step wise atomic motion.

Creep

Materials that are placed in service at elevated temperatures and exposed to static mechanical stresses. For metals it becomes important only for temperatures greater than 0.4Tm (Tm=absolute melting temperature.) Amorphous polymers such as plastics and rubbers are especially sensitive to creep.

Toughness

Mechanical property of a material that indicates the ability of the material to resist high energy, high impact, and shock loads before it fractures. It is temperature dependent and has high strength and high ductility.

Steady State Diffusion

Occurs if the flux does not change with time (independent of time). Diffusion flux is proportional to the negative of the concentration gradient according to Fick's first law. The driving force for steady-state diffusion is the concentration gradient (slope on concentration vs position plot).

Plastic Deformation

Occurs when a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is released.

Elastic Deformation

Occurs when an object changes shape because stress is being applied, but snaps back into shape when the stress is removed.

Tensile Strength

Or ultimate strength, is the largest unit stress a material can achieve in a tensile test before the material fractures. As temperature increases, tensile strength decreases.

Cold Working

Plastic deformation of a metal below its recrystallization temperature.

Proportional Limit

Point at which the deformation is no longer directly proportional to the applied force. Hooke's Law no longer applies. This position at this point is difficult to measure precisely.

Strain Hardening

Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is plastically deformed. Degree of plastic deformation may be expressed as percent cold work, which depends on original and deformed cross-sectional areas. Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work. During plastic deformation dislocation density increases, the average distance between adjacent dislocations decreases, and dislocation mobility becomes more restricted; thus the metal becomes harder and stronger.

Resiliance

The capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered.

Vacancy Diffusion

The interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy

Factors That Influence Diffusion

The magnitude of the diffusion coefficient is indicative of the rate of atomic motion and depends on both host and diffusing species as well as on TEMPERATURE.

Fatigue Strength

The maximum stress level that a material can sustain without failing for some specified number of cycles.

Poisson's Ratio

The ratio of the transverse strain over axial strain. For many metals, the value of Poisson's ratio is about .33 and polymers is .4.

Recrystallization Temperature

The recrystallization temperature of a metal alloy is that temperature at which recrystallization reaches completion in one hour. Two factors that influence the recrystallization temperature are percent cold work and impurity content; recrystallization temperature diminishes with increasing percent cold wok, and it rises with increasing concentrations of impurities. For pure metals recrstallization temp is 0.4Tm (Tm=absolute melting temp)

Solid Solution Strengthening

The strength and hardness of a metal increase with increase of concentration of impurity atoms in a solid solution (both substitutional and interstitial). Solid solution strengthening results from lattice strain interactions between impurity atoms and dislocations; these interactions produce a diminishment in dislocation mobility.

Yield Strength

The stress a material can withstand without permanent deformation. For most materials, yield strength is determined from a stress-strain plot using the .002 strain offset technique. As temperature increases, yield strength decreases.

Yielding

The stress level at which plastic deformation begins.

Fatigue Life

The total number of stress cycles that will cause fatigue failure at some specified stress amplitude. You can extend fatigue life by reducing the mean stress level, eliminating sharp surface discontinuities, improving the surface finish by polishing, imposing surface residual compressive stresses by shot peening, and case hardening by using a carburizing or nitriding process.

Nonsteady-State Diffusion

There is a net accumulation or depletion of diffusing species and the flux is dependent on time.

Thermal Fatigue

Thermal stresses may be induced in components that are exposed to elevated temperature fluctuations and when thermal expansion/contraction is restrained.

Diffusion Mechanisms

Vacancy Diffusion Interstitial Diffusion

Modulus of Elasticity

a coefficient of elasticity of a material, expressing the ratio between a unit stress and the corresponding unit strain caused by the stress, as derived from Hooke's law and represented by the slope of the straight-line portion of the stress-strain diagram (coefficient of elasticity). A materials stiffness/strength is based on E.

Interstitial Diffusion

atoms migrate from an interstitial position to a neighboring one that is empty , this occurs more rapidly than vacancy diffusion because interstitial atoms are smaller and more mobile.

Diffusion Flux

how fast diffusion occurs. The mass diffusing through a perpendicular to a unit cross sectional area of solid per unit of time. J=M/(At)

Stress

internal resistance offered by a unit area of a material to an externally applied load or force. Stress(sigma)=Force/Area

Strain

physical-measureable deformation.

Carburizing

produces a high carbon layer on the surface of a metal by exposing the metal to a carbon-containing atmosphere at elevated temperatures, such that carbon atoms diffuse into the surface of the metal.

Modulus of Resilience

ratio of the elastic energy to the strain at yielding. Determines how much energy will be used for deformation and how much will be translated to motion.

Anelasticity

time-dependent elastic behavior (ie. time to original state). Elastic deformation will continue after stress application, and upon load release some finite time is required for complete recovery.